This invention relates generally to a handle apparatus for a power-assist device where it is desired to cause a motor to position a heavy object in response to a small manual force being applied to the handle. The invention relates particularly to the handle having force sensors which provide a signal to a variable speed motor proportional to the manual force applied to the handle.
Although the device is useful in many kinds of machines, a typical use of the new handle is in an x-ray table which is equipped with a spot-film device. A spot-film device includes a carriage which is mounted in the body of an x-ray table for being moved longitudinally thereof. Supported on the carriage above the table is an enclosure which may be adated for accommodating a film cassette and an x-ray image intensifier. The enclosure is usually mounted on the carriage in such manner that it may be shifted to various positions relative to the x-ray table top.
It is customary to have a manually engageable handle attached to the enclosure. The operator may then grasp the handle and push or pull the enclosure to position it as required. The handle enables the operator to apply force in a longitudinal direction for activating an electric motor that drives the heavy carriage to its desired longitudinal positions. Additional electric motor drives can be provided for moving the carriage to desired vertical and lateral positions.
In some prior art x-ray tables, the operating handle is mounted for pivotting or sliding through a perceptible distance so that when a force is applied, the handle may actuate potentiometers or switches which select motor speed and direction for longitudinal positioning of the spot-film device. In using this prior art system, the operator was deprived of the naturally expected feeling of proportionality between the magnitude of the manually applied force and the rate and direction in which the spot-film device moves. Despite extensive efforts at refinement, in proportional systems where potentiometers or switches are used to initiate driving action, the operator does not get a true sense or feeling of the massive component moving in proportion to the force which is being applied.
The prior art has attempted to proportion the manual force applied to a handle and the rate of the movement by a motor system as shown in U.S. Pat. No. 3,866,048 to Gieschen, et al, which employs a piezo-electric crystal in conjunction with movement of a handle. A piezo-electric crystal is extremely expensive for this application. The crystal is shown between the handle and the equipment with the crystal receiving a force through a bulky lever and linkage mechanism. It is very difficult to isolate force in each separate axis of multi-axis crystals, so there is frequently cross talk between the crystals.
Another prior art system is shown in copending U.S. patent application Ser. No. 733,644 to Pury, et al, now U.S. Pat No. 4,107,590, assigned to the assignee of the present invention, which incorporates a handle operating through a flexure member containing strain gauge transducers to produce a signal which drives a variable speed motor proportional to the forces applied to the handle. The system can provide excellent proportional performance for a specific device; however, the strain gauge transducers are very expensive and delicate, and require complex chemical bonding to a heavy flexure member. The quality of bonding varies from assembly to assembly and results in variable sensitivity and response to an applied force.
The function of the strain gauge and crystal transducers require relatively heavy members for operation and, therefore, requires error correction for the forces of the weight of the handle itself in certain positions.
The function of the strain gauges results in cross talk between adjacent gauges which are intended to respond independently to forces along different axes.
A particular problem with strain gauge transducers is the considerable signal drift during warm-up and during changes in ambient temperature. The assemblies are difficult to calibrate and do not have consistent performance.
Accordingly, one object of the present invention is to provide a handle for a power-assist device which uses inexpensive components which can be easily and repeatably installed.
Another object of the invention is to provide a handle which is compact, housing the force transducers within the handle itself, and which is lightweight requiring no error correction for position.
Still a further object of the present invention is to provide a handle which can be easily zeroed and which has repeatable sensitivity over an extended life during changes in ambient conditions.